1. Field of the Invention
The present invention relates to a simply structured flowmeter that is excellent in response and measurement accuracy and to a flow control device using the flowmeter.
2. Description of the Related Art
Unexamined Japanese Patent Publication No. 2004-3887 discloses, as a flowmeter for accurately detecting a gas (fluid) flow rate, a thermal mass flowmeter that uses a flow rate sensor in which two temperature sensitive elements are placed with a heating element interposed between them in a gas flowing direction, and measures a mass flow rate of gas from temperature difference detected by temperature sensitive elements. As to such a flow rate sensor, for example, Unexamined Japanese Patent Publication No. 2006-118929 describes a flow rate sensor having a device structure in which a heating resistive element made of platinum or the like and two temperature-sensitive resistive elements are aligned on a choke made of silicon nitride, silicon oxide or the like, which is formed in the surface of a silicon chip.
Schematically speaking, a thermal mass flowmeter of this type is so constructed that a flow rate sensor 3 having the above device structure is disposed in the inner wall surface of a flowmeter body 2 in which a flow channel 1 is formed as shown in FIG. 20 so that the gas may flow along the flow rate sensor 3. The thermal mass flowmeter thus constructed is capable of detecting a flow rate in a minimal flow rate area, has wide flow-rate detection width (measurement range), and is excellent in detection response.
On the other hand, this thermal mass flowmeter has a drawback that, if a dilatational wave (compression wave) is created in the gas existing in the flow channel, for example, due to an impact applied to the flowmeter body 2, pipes of the flowmeter body 2 and the like, the dilatational wave is transmitted to the flow rate sensor 3. As a result, the flowmeter then detects the dilatational wave as a flow rate even when there is no gas flow. In short, because of its high detection sensitivity, the flow rate sensor 3 detects a transfer of gas molecules, which is caused by the dilatational wave, as if it is a gas flow. Therefore, the dilatational wave created in gas degenerates the measurement accuracy and measurement repeatability of the thermal mass flowmeter.
This is a serious issue especially when a minimal flow rate is to be measured. In general, a change of a sensor output which is caused by the dilatational wave of several hundred HZ or less is more significant than a sensor output corresponding to the minimal flow rate to be detected by the flow rate sensor 3. The minimal flow rate (signal component) that is aimed to be measured is prone to fade into disturbance (noise component) caused by the dilatational wave. As a result, the measurement accuracy and the measurement repeatability are degenerated.
As a device for attenuating the dilatational wave or the like, a single hollow silencer illustrated in FIG. 21 has been well known. This silencer is formed by reducing the diameters of the inlet and the outlet of the flow channel 1 to make the inlet and the outlet into narrowed portions 4 and 5, and thereby forming a spatial filter in the hollow (expanded portion) 6 between the narrowed portions 4 and 5. However, a silencer of this type effectively functions only with respect to certain frequency components because a sound deadening amount (attenuation amount) is frequency dependent as shown in FIG. 22. It is then very difficult to design the silencer for coping with the dilatational wave whose frequency is unspecific. It is especially difficult to deaden (attenuate) sounds at a low frequency of 100 HZ or less. Therefore, the silencer is not capable of effectively solving the above-described issue.